Our studies are designed to improve the use of platelet transfusions in thrombocytopenic patients, particularly those with acute leukemia. Patients at Children's Hospital are randomized between two groups, prophylactic and therapeutic. Patients in the prophylactic group receive transfusions whenever the platelet count falls below 20,000 per mm3; those in the therapeutic group receive transfusions only for specific clinical indications. The long-term clinical courses of these two groups are being compared. We are also studying methods to improve the liquid storage of platelets. Specifically, bags constructed of polyethylene appear to be markedly superior to those constructed of polyvinylchloride for storage at 22 degrees C due to their increased premeability to oxygen. In the coming year, we plan studies of platelets stored in high oxygen atmospheres, as well as the effect of a variety of methods of agitation. We plan to define more precisely the functional lesion which develops after centrifugation and after storage at 22 degrees C.